The Chloride Homeostasis of CA3 Hippocampal Neurons Is Not Altered in Fully Symptomatic Mice.

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the gene. Mouse models of RTT show reduced expression of the cation-chloride cotransporter KCC2 and altered chloride homeostasis at presymptomatic stages. However, whether these alterations persist to late symptomatic stages has not been studied. Here we assess KCC2 and NKCC1 expressions and chloride homeostasis in the hippocampus of early [postnatal (P) day 30-35] and late (P50-60) symptomatic male mice. We found (i) no difference in the relative amount, but an over-phosphorylation, of KCC2 and NKCC1 between wild-type (WT) and hippocampi and (ii) no difference in the inhibitory strength, nor reversal potential, of GABA -receptor-mediated responses in CA3 pyramidal neurons compared to WT at any stages studied. Altogether, these data indicate the presence of a functional chloride extrusion mechanism in CA3 pyramidal neurons at symptomatic stages.